Transition is an important phenomenon in large scale, commercial, wind tunnel testing at low speeds because it is an excellent indicator of an airfoil performance. It is difficult to estimate transition through numerical techniques because of the complex nature of viscous flow. Therefore experimental techniques can be essential. Over the transition region the rate of heat transfer shows significant increases which can be detected using infrared thermography. This technique has been used predominantly at high speeds, on small models made of insulated materials, and for short test runs. Large scale testing has not been widely undertaken because the high sensitivity of transition to external factors makes it difficult to detect. The present study records the process undertaken to develop, implement and validate a transition detection system for continual use in the Virginia Tech Stability Wind Tunnel: a low speed, commercial wind tunnel where large, aluminium models are tested. The final system developed comprises of two high resolution FLIR A655sc infrared cameras; four 63.5-mm diameter circular windows; aluminium models covered in 0.8-mm silicone rubber insulation and a top layer of ConTact© paper; and a series of 25.4-mm wide rubber silicone fiberglass insulated heaters mounted inside the model and controlled externally by experimenters. This system produces images or videos of the model and the associated transition location, which is later extracted through image processing methods to give a final transition location in percentage chord. The system was validated using two DU96-W-180 airfoils of different chord lengths in the Virginia Tech Stability Wind Tunnel, each tested two months apart. The system proved to be robust and efficient, while not affecting the airfoil performance or any other system in use in the wind tunnel. Transition results produced by the system were compared to measurements obtained from pressure data and stethoscope tests as well as the numerical predictions of XFOIL. The transition results from all four methods showed excellent agreement with each other for the two models, for at least two Reynolds numbers and for several angles of attack on both suction and pressure side of the model. The agreement of data obtained under such different conditions and at different times suggests that the infrared thermography system efficiently and accurately detects transition for large aluminium models at low speeds. / Master of Science

This study is undertaken to characterize the aerodynamic behavior of Kevlar-wall test sections and specifically those containing two-dimensional, lifting models. The performance of the Kevlar-wall test section can be evaluated against the standard of the hard-wall test section, which in the case of the Stability Wind Tunnel (SWT) at Virginia Tech can be alternately installed or replaced by the Kevlar-wall test section. As a first step towards the evaluation of the Kevlar-wall test section aerodynamics, a validation of the hard-wall test section at the SWT is performed, in part by comparing data from NACA 0012 airfoil sections tested at the SWT with those tested at several other reliable facilities. The hard-wall test section showing good merit, back-to-back tests with three different airfoils are carried out in the SWT's hard-wall and Kevlar-wall test sections. Kevlar-wall data is corrected for wall interference with a panel method simulation that simulates the unique boundary conditions of Kevlar-wall test sections including the Kevlar porosity, wall deflection, and presence of the anechoic chambers on either side of the walls. Novel measurements of the boundary conditions are made during the Kevlar-wall tests to validate the panel method simulation. Finally, sensitivity studies on the input parameters of the panel method simulation are conducted. The work included in this study encompasses a wide range of issues related to Kevlar-wall as well as hard-wall tunnels and brings to light many details of the performance of such test sections. / Master of Science

Two experiments were conducted to investigate Adaptive Noise Cancelling and Cepstrum echo removal post-processing techniques on acoustic data from a linear microphone array in an anechoic chamber. A point source speaker driven with white noise was used as the primary signal. The first experiment included a background speaker to provide interference noise at three different Signal-to-Noise Ratios to simulate noise propagating down a wind tunnel circuit. The second experiment contained only the primary source and the wedges were removed from the floor to simulate reflections found in a wind tunnel environment. The techniques were applicable to both signal microphone and array analysis. The Adaptive Noise Cancellation proved successful in its task of removing the background noise from the microphone signals at SNRs as low as -20 dB. The recovered signals were then used for array processing. A simulation reflection case was analyzed with the Cepstral technique. Accurate removal of the reflection effects was achieved in recovering both magnitude and phase of the direct signal. Experimental data resulted in Cepstral features that caused errors in phase accuracy. A simple phase correction procedure was proposed for this data, but in general it appears that the Cepstral technique is and would be not well suited for all experimental data. / Master of Science

On-blade hot wire anemometry measurements as well as far field sound measurements at several receiving angles have been previously made for a rotor partially embedded in a boundary layer. The inflow distortion effect on the rotor angle of attack distribution was determined directly from the on-blade measurements, and was found to minimally affect the angle of attack at the blade tips and lower the angle attack in the rotor disk plane as the radial location moves towards the hub. A narrow, sharp increase in angle of attack as the rotor blades approached the wall was also observed, indicating blade interaction with flow reversal. The haystacking pattern, or spectral humps that appear at multiples of the blade passage frequency, was studied for a wide range of advance ratios. At high advance ratios, evidence of vortex shedding from the blade trailing edges was observed. For low advance ratios, the haystacks narrowed, became more symmetric and increased in number. A method of determining the average acoustic signature of an eddy passage through a rotor was developed from time delay aligning multiple microphone signals and eddy passages detected using the continuous wavelet transform. It was found that the eddy passage signatures were similar to a cosine wave with a Gaussian window. It was also found that normalized timescales obtained directly from the eddy passage signatures remained somewhat constant with advance ratio, but increases slightly for fixed free stream velocities with increasing rotor RPM. For advance ratios less than 0.6, the eddy passage signatures were dominated by a tonal component due to rotor ingestion of misaligned flow caused by a boundary layer separation at the wall. This indicates that flow reversal known as the Pirouette Effect is interacting with the rotor blades. / Master of Science

In 1998, the Rome Statute established the International Criminal Court (ICC) to end impunity for violators of international human rights law. As the ICC is opening criminal investigations for the first time in its existence, it is important to determine the legitimacy of the young institution in order to understand its importance in international politics and international legal precedence. These first cases can be used to illustrate that while some fears might be misplaced, others are sadly realized. Especially through the criminal investigation processes in Darfur, the ICC has acted responsibly and has not violated its founding principles or Sudan's sovereignty. Conversely, ICC intervention in Uganda has created a political situation that pits the prospect of peace against the pursuit of justice. If the ICC is able to prove that it is responsible in its judicial processes, it will likely become a legitimized institution. An increased role by the international community in ICC affairs would also bring a level of comfort and transparency that has not yet been realized. Furthermore, as individual states begin to use diplomatic means to enforce the norms of international human rights, the court might be used infrequently, and only when it is critical in the pursuit of justice. Despite the difficulties. faced by the ICC, it has the potential to gain legitimacy and become a recognizable player on the international political scene.

<p dir="ltr">The dynamics and characteristics of a high-speed compressible turbulent boundary layer or shear layer have significant effects on separation, heating, shockwave boundary layer interactions, effectiveness of control surfaces, and ultimately the performance of supersonic / hypersonic vehicles. Experimental data with high spatiotemporal resolution and low uncertainty is necessary for understanding complex flow physics and validating computational models. </p><p dir="ltr">Background oriented schlieren (BOS) is a technique derived from traditional schlieren imaging to provide whole-field, quantitative density gradient measurements with a simplistic setup at the expense of reduced spatial resolution and increased uncertainty. The majority of BOS applications focus on low-speed flows with an entocentric optical setup which causes low depth-of-field, wall-blurring, and perspective error issues, making conventional BOS not suitable for high-speed compressible turbulent flow settings. Additionally, despite the widespread adoption of BOS, it has primarily been used as an alternative visualization technique to traditional schlieren imaging and thus the quantitative capabilities of BOS are left under-exploited.</p><p dir="ltr">The workflow of BOS consists of image acquisition, displacement estimation, and integration of the density gradient field. The work presented in this thesis improves the image acquisition and displacement estimation of the BOS workflow by implementing a telecentric optical system and conducting a comprehensive comparison and optimization of several state-of-the-art displacement estimation techniques. Experimental results for a Mach 2 turbulent boundary layer exhibit high spatiotemporal resolution and low uncertainties and are compared against high-fidelity computational results for validation. This work also focuses on the development of BOS velocimetry capabilities, by leveraging ray tracing simulations of an LES turbulent shear layer. Overall this dissertation advances the accuracy, precision, spatial resolution, and capabilities of BOS for fluid dynamic applications relevant to defense and propulsion.</p>

Wall shear stress is one of the most essential scaling parameters used in fluid dynamics. It is significant because it helps us compare results in different experimental studies. The accurate measurements of wall shear stress will be instrumental in improving the existing empirical models and validating CFD models. Wall shear stress is also vital in improving fuel efficiency, heat transfer efficiency, and aerodynamic efficiency in real-world applications. This work discusses the design and implementation of a prototype floating element balance — a direct method of wall shear measurement. The direct measurement methods are robust and can significantly improve the validity of experimentation when perfected. In this work, a prototype floating element balance is designed and developed to estimate the wall shear stress in a smooth wall pilot facility to assess its feasibility for large-scale development. The floating element balance utilizes a strain gauge to estimate the wall shear stress. The preliminary tests show promising results, revealing potential design improvements. A strain measurement study is conducted to investigate the force-strain relationship and the reliability of the balance, which highlights the long-term stability and consistency in the strain measurement. However, further investigations are required into the drift response of the floating element balance. The strain measurements are also employed to calibrate the balance using a linear curve fit with a coefficient of determination of R^2 = 0.99, indicating a satisfactory linear estimation. / Master of Science / Drag is a phenomenon that occurs when the surface of a solid body interacts with a fluid. In fluid mechanics, there are two fundamental types of drag forces: pressure and skin friction. Pressure drag occurs due to the shape of the body, creating a pressure difference across the body, while skin friction drag arises due to the dominant viscous nature of the fluid. Understanding these forces is vital in improving the aerodynamic efficiency of various devices. Also, these forces play an essential role in the fuel efficiency and performance of the vehicles. The measurement of pressure drag is relatively straightforward compared to the skin friction drag. However, the measurement of skin friction drag can pose a challenge due to its smaller magnitude than the pressure drag. The importance of skin friction is due to its physical properties, which allow us to compare different experimental results and understand details about the turbulence in the flow. Also, accurate information on skin friction would improve existing relationships in fluid mechanics, and this information is also utilized to validate mathematical models in fluid mechanics. This work presents the design and implementation of a prototype used to estimate skin friction in a smooth wall facility using a novel and robust measurement known as floating element skin friction balance. Preliminary tests are conducted to assess the viability of the floating element balance for a large-scale development, which shows promising results while underlining some inherent limitations in the design and performance.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / The aerodynamic performance of an existing wind turbine blade optimised for low wind speed conditions is investigated. The aerodynamic characteristics of four span locations are determined from surface pressure measurements and wake surveys with a traversed five-hole probe performed in a low speed wind tunnel for chord Reynolds numbers ranging from 360,000 - 640,000. Two-dimensional modelling of the wind tunnel tests is performed with the commercial computational fluid dynamics code FLUENT. The predictive accuracies of five eddy-viscosity turbulence models are compared. The computational results are compared to each other and experimental data. It is found that agreement between computational and experimental results varies with turbulence model. For lower Reynolds numbers, the Transitional-SST turbulence model accurately predicted the presence of laminar separation bubbles and was found to be superior to the fully turbulent models considered. This highlighted the importance of transitional modelling at lower Reynolds numbers. With increasing angles of attack the bubbles were found to move towards the leading edge and decrease in length. This was validated with experimental data. For the tip blade section, computations implementing the k-ε realizable turbulence model best predicted experimental data. The two-dimensional panel method code, XFOIL, was found to be optimistic with significantly higher lift-to-drag ratios than measured. Three-dimensional modelling of the rotating wind turbine rotor is performed with the commercial computational fluid dynamics code NUMECA. The Coefficient of Power (Cp) predicted varies from 0.440 to 0.565 depending on the turbulence model. Sectional airfoil characteristics are extracted from these computations and compared to two-dimensional airfoil characteristics. Separation was found to be suppressed for the rotating case. A lower limit of 0.481 for Cp is proposed based on the experimental data. / Centre for Renewable and Sustainable Energy Studies

Aerodynamic analysis is an important area of survivability studies. There is a desire to be able to predict the aerodynamic effects of a given damage scenario on an aircraft wing with minimal wind tunnel testing or computational simulations. Due to the limited nature of previous studies, this has not generally been possible. The original contribution of this thesis is a predictive technique developed to estimate the aerodynamic effects of a simulated battle damage hole on an aircraft wing, resulting from a range of attack directions. This technique was successfully validated against experimental data. Testing under two-dimensional conditions was undertaken on a NASA LS(1)-0417MOD aerofoil at a Reynolds number of 500,000. This project simulates the effect of attack direction by varying the offset between upper and lower surface damage holes in both chordwise and spanwise directions. Damage was modelled using circular holes. Lift, drag and pitching moment coefficients were measured and supplemented with surface flow visualisation and surface pressure measurements. Coefficient increments, defined as the difference between the damage cases and a datum undamaged case were used to quantify the effects of the damage, with the performance qualified in terms of weak and strong jets. Weak jets were found to have little effect on the flow and aerodynamic properties, while strong jets caused significant disruption. The effects increased in magnitude with hole size, incidence and proximity of the upper surface hole to the pressure peak. Spanwise offset on the holes had little effect on the jet strength but introduced asymmetry into the surface flow. This effect was found to be due to the behaviour of the flow within the cavity. Three-dimensional testing was undertaken at a Reynolds number of 1,000,000 on a half wing model in order to investigate any changes in the aerodynamic characteristics of the damage when applied to a more representative aircraft wing. The higher Reynolds number exploited the larger wind tunnel working section and provided a value more representative of typical unmanned aerial vehicles. As the damage was moved towards the tip its effects were lessened and the transition from weak jet to strong jet delayed. Spanwise pressure variation from the tip also introduced asymmetry into the jet s surface flow features. Plotting coefficient increments for all attack directions against the pressure coefficient difference between upper and lower surfaces from an undamaged wing, across the equivalent damage hole region highlighted significant trends, which were used as the basis of a predictive technique for a range of hole sizes and attack directions. The validity of the technique was assessed by predicting a previously untested damage case and comparing it against subsequent wind tunnel tests. The results from this validation proved encouraging.

Lors des rentrées atmosphériques, les processus thermochimiques hors équilibre dans la couche de choc limitent la fiabilité des prédictions aérothermiques. Afin d'améliorer l'exactitude de ces prévisions, des modèles cinétiques sont actuellement développés. Ces modèles sont expérimentalement évalués à l'aide d'expériences dans lesquelles un départ à l'équilibre thermodynamique est caractérisé. Pour cette raison, le présent travail est consacré à la caractérisation du déséquilibre thermodynamique au sein d'écoulements réactifs à haute enthalpie. La plupart des études expérimentales dédiées à la validation de modèles cinétiques à haute température emploient des installations communément appelées tubes à choc. Nous évaluons ici la possibilité de générer un départ significatif à l'équilibre thermodynamique dans des écoulements plasma stationnaires, incluant des jets supersoniques dans lesquels le déséquilibre vibrationnel est fortement attendu. Des diagnostics spectroscopiques appropriés ont été appliqués, permettant de futures comparaisons avec des descriptions microscopiques issue de modèles théoriques. / During planetary atmospheric entries, thermochemical non-equilibrium processes in the shock layer limit the reliability of aerothermal environment prediction. To improve prediction accuracy, non-equilibrium kinetic models are being developed. These models are experimentally assessed through the comparison with well characterized non-equilibrium experiments. For this purpose, the present work is dedicated to the thermodynamic characterization of non-equilibrium in high enthalpy reactive flows. Conversely to common studies that employ short duration facilities to investigate shock layer kinetics, we will assess the possibility of producing significant departure from equilibrium using radio-frequency and microwave stationary plasma flows, including supersonic plasma flows where vibrational non-equilibrium is strongly expected. Suitable spectroscopic diagnostics have been applied allowing future comparisons to be made between the microscopic description of the experiments and theoretical non-equilibrium models.